chthonic

Something chthonic dwells beneath the earth. This word usually refers to mythological creatures, but you could also refer to your creepy basement as chthonic.

Ever heard about the mole men who live underneath the ground in tunnels? Or the mutants who live in the sewers? Or even the Fraggles? Of course, those critters are imaginary, but they're examples of chthonic creatures: beings who live under the surface of the earth. Chthonic beasts are more likely to be demons than angels, so this adjective has a hellish aspect. Many myths feature chthonic creatures.